This invention relates to hydrophone preamplifiers and especially to a miniature hydrophone preamplifier suitable for use with remote hydrophone arrays.
Most hydrophone preamplifiers used in arrays are not capable of driving extremely long lines, although some contain low-output-impedance, line-driver circuits. When such a preamplifier is used with a two-wire cable, separation of signal and DC power becomes a problem. The designer is forced to either specify a high-impedance load or deliver high current to a low-impedance load. High-impedance loads can cause excessive electrical crosstalk between array outputs, and high-current low-impedance loads waste power. Other preamplifier designs avoid these difficulties by using an output current driver rather than a voltage driver. A low-impedance load can then be used with a small, controlled load current. When such a preamplifier is used with a two-wire cable, however, separation of DC current to and AC signal current from the device is again a problem. This is because the preamplifier supply current is not fixed but varies slightly with the signal. This unwanted ripple in the supply current adds to the AC signal current resulting in an inaccurate output.
Moreover, providing calibration circuitry for array preamplifiers typically requires a separate wire pair for each preamplifier. In an array, this means many more conductors in the cable. Some preamplifier designs avoid this by providing built-in oscillators. These oscillators produce local calibration signals for their respective preamplifiers. But then additional control lines must be provided to turn the oscillators on or off. Other designs use one common calibration line to send a calibration signal to all array preamplifiers. But any common circuit, such as this, creates potential cross-talk between units when they are operating normally with acoustic input signals. To avoid crosstalk via the common calibration line, complete isolation is required between calibration circuits.
Some array designs which use a single-input calibration cable suffer from another shortcoming: due to cable resistance, each calibration circuit along the array receives a slightly different input calibration signal level. Other calibration circuit designs cause the entire preamplifier to fail when the calibration input opens or shorts. All of these calibration, as well as output drive, problems are solved by the present invention.